1. Field of the Invention
This invention relates to a magnetic recording medium, and particularly to a method for manufacturing a magnetic paint for forming a magnetic recording film on a magnetic recording medium, and a magnetic recording medium suitable for a high recording density magnetic disk having distinguished electrical characteristics and reliability, prepared by using the magnetic paint.
2. Description of the Related Art
Methods for preparing a magnetic recording medium by applying a magnetic paint comprising a magnetic powder dispersed in a polymer binder including epoxy resin, etc., have been proposed (e.g., Japanese Patent Publication No. 55-816), and methods for manufacturing a magnetic paint by grinding a magnetic powder together with epoxy resin, etc., are disclosed in Japanese Patent Publication No. 57-40566 and Japanese Patent Application Kokai (Laid-open) No. 56-100871, where the magnetic powder is ground together with a resin solution comprising epoxy resin, etc., dissolved in cyclohexanone under a high shear stress to prepare a magnetic paint, and then a substrate is coated with the thus obtained magnetic paint to prepare a magnetic recording medium. However, in the prior art, no consideration has been given to thorough mixing of the magnetic powder with epoxy resin as a binder for the dispersion of the magnetic powder before grinding. Thus, in the magnetic paints obtained according to the prior art methods, a small amount of the resin solution as added at the grinding is locally absorbed into a portion of the magnetic powder, and the grinding is carried out in the locally absorbed state. That is, it has been quite difficult due to the nature of grinding to obtain a magnetic paint of such a tactoid structure that the magnetic powder is uniformly dispersed throughout the mixture. Consequently, it has been impossible to form a thin film having a film thickness of not more than about 0.9 .mu.m as a magnetic recording film from the magnetic paint manufactured according to the prior art methods. Also, the film, even if formed, has a surface roughness of about 0.08 .mu.m Ra before surface finishing.
With a keen demand for magnetic disks having higher recording density and consequent smaller film thickness, the surface finishing work is inevitably increased, resulting not only in an increased surface finishing time, but also in an increased occurrence of surface defects such as scratches on the film surface and a consequent increase in electrical defects.
The magnetic paint manufactured according to the said prior art methods has an insufficient dispersion of magnetic powder throughout the magnetic paint, and it is substantially impossible to form a thin film having a film thickness of not more than 0.9 .mu.m from such a magnetic paint, and the film, even if formed, has a surface roughness of about 0.08 .mu.m Ra before the surface finishing.
Furthermore, in high density recording magnetic disk drives which are now widely used, a thin film magnetic head is used as a magnetic head, and thus a magnetic disk medium having a large film thickness has another problem such as a failure to thoroughly write signals. Usually, a magnetic disk medium has a magnetic thin film so formed on a disk plate as a substrate as to have a smaller film thickness on the inner peripheral side and a larger film thickness on the outer peripheral side. Thus, the aforementioned problem concerning the thin film magnetic head usually occurs on the outer peripheral side of the magnetic disk medium. To solve the problem, it is necessary that the magnetic disk medium has a magnetic thin film having a smaller film thickness on the outer peripheral side. However up to now no method other than intensifying the film finishing work to the film surface on the outer peripheral side more than that to the film surface on the inner peripheral side has been taken. As a result, the surface finishing time and frequency of scratch occurrence, and consequently electrical defects are inevitably increased. The increase in the electrical defects may cause a maloperation in the magnetic recording --reproducing apparatus, and scratches developed on the film surface also damage the disk-facing side of the magnetic head and cause head crushes.